Flexible tube-form heating wires, etched aluminum sheet-form heaters, conductive fabrics or the like have been conventionally applied to sheet-form electric warmers such as electric carpets, electric blankets, floor heating apparatus, panel heaters and the like. As temperature sensors detecting the temperature of these electric warmers, temperature-detecting wires using thermosensitive polymeric compounds, thermal heating wires, metal resistance wires and the like have been employed.
When a thermosensitive polymeric compound, which is a high impedance material of 10.sup.9 -10.sup.6 .OMEGA..multidot.cm, is used for the temperature sensor, it should be used in the form of a temperature-detecting wire or an etched aluminum sheet-form heater to lower the impedance significantly. As a result, no noise malfunction is generated in the temperature sensor. The temperature-detecting wire and the thermal heating wire are electric wires having doubly wound spiral electrodes, and layers of thermosensitive polymeric compounds have a parallel structure in a wire-length direction and are lowered in impedance. The etched aluminum sheet-form heater, on the other hand, comprises a polymeric temperature-sensing layer sandwiched between two sheets of etched aluminum heaters, and the impedance of the thermosensitive polymeric layer varies with temperature.
The temperature-sensing wires using the thermosensitive polymeric compound are of two different forms, including a two-wire system consisting of the temperature-sensing wire and the heating wire, and a one-wire system in which one electrode wire in the temperature-sensing wire is used for passing electric current and generating heat.
A PTC (positive temperature coefficient) heating resistor, having a large positive temperature coefficient of resistance, constantly controls its own temperature when a constant voltage is applied to a pair of electrodes, which gives uniform temperature distribution. Therefore, the PTC heating resistor is used as a temperature self-controlling heater such as a flat cable having parallel electrode wires, a tape heater having sandwiched electrodes or a sheet-form heater formed on heat-diffusion plates.
However, the temperature uniformity of the tube-form heating wires is low since they are distributed with space in between them. In addition, the temperature of locally overheated sections of the wire becomes high. When a heating wire is broken at one location, the whole heater stops heating.
Etched aluminum sheet-form heaters, on the other hand, have uniform temperature distributions. However, the sheet-form heater is likely to be broken when it is bent.
Since conventional conductive fabrics do not have unidirectional conductivity, high voltage is likely to be applied around the broken sections, thus promoting local overheatings. As a result, conventional conductive fabrics may cause a fire.
A PTC heating resistor comprises a PTC resistance sheet with parallel electrode wires. Therefore, due to the flow of heavy electric current in the parallel electrode wires, overheating occurs by an electric arc or the like at the section of the breakage of the electrode wire. Moreover, if the PTC heating resistor is not applied on a heat-diffusion plate, high voltage is likely to be concentrated locally at the high temperature section of the PTC resistance sheet by a positive feedback. As a result, heat does not generate evenly, and the materials of the resistor deteriorate, thus causing accidents.